Further Studies Involving Cationic Activities in Systems of Plant Roots

Cationic activities were determined by means of clay membrane electrodes, and mean free bonding energies were computed for Ca and K in systems of plant roots representing five different plant species. The study involved measurements in both homoionic and biionic systems of roots from Reed canary grass, red top, oats, alfalfa, and buckwheat. When all cation saturations of the roots studied are considered, results indicate that on a molar basis Ca is usually bonded to plant roots more strongly than is K. Using the homoionic systems at 100% saturation as a basis for comparison, buckwheat roots were found to have bonded Ca 3.3 times as strongly as K. Reed canary grass roots under the same conditions, however, held K equally as strongly as Ca. Roots from the other plant species were intermediate between these extremes in relative bonding energies for these two cations. Varietal differences in the relative bonding energies were also noted. The ratio of the mean free bonding energies of the cations in the homoionic systems at 100% saturation was found not to be closely related to the cation exchange capacity of the roots. This suggests that the root exchange capacity alone will not be a reliable guide in predicting relative uptake of Ca and K by different crops. It is suggested that buckwheat's well known ability to utilize phosphates of low solubility may be related to the relatively high mean free bonding energy for Ca by its roots. Such a high relative bonding energy might well be the weathering force necessary to release the phosphate from the otherwise unavailable form.